fix(roslyn): install missing System.Runtime.CompilerServices.Unsafe and surface inner errors

The RoslynInstaller downloads only 4 NuGet packages but Microsoft.CodeAnalysis 4.12.0
on netstandard2.0 also references System.Runtime.CompilerServices.Unsafe v6.0.0.0,
which is NOT what Unity ships (Unity bundles v4.x). The reference is unresolved at
runtime, so Roslyn's StringTable static cctor throws FileNotFoundException, which
in turn poisons CSharpSyntaxTree's cctor: every parse / compile attempt then
throws TypeInitializationException.

The error is invisible because RoslynCompiler.Compile's catch block only logs
e.Message, and for TargetInvocationException that string is the generic
"Exception has been thrown by the target of an invocation." — the real cause
in InnerException is silently dropped.

Repro:
  1. Trigger Tools > MCP for Unity > Install Roslyn on a fresh project.
  2. Ask the MCP execute_code tool to compile any Roslyn-only snippet.
  3. Observe: "Compilation failed: Roslyn compilation error: Exception has been
     thrown by the target of an invocation." — with no further detail.
  4. Drilling via reflection reveals:
       TypeInitializationException for CSharpSyntaxTree
        └─ TypeInitializationException for Roslyn.Utilities.StringTable
            └─ FileNotFoundException: Could not load file or assembly
                'System.Runtime.CompilerServices.Unsafe, Version=6.0.0.0'

Fix:
  * RoslynInstaller.NuGetEntries: add the missing dependency so a fresh install
    drops all 5 DLLs into Assets/Plugins/Roslyn/.
  * RoslynCompiler.Compile catch: walk the InnerException chain so a future
    bootstrap regression surfaces the actual cause (type + message) instead of
    the generic invocation wrapper.

Verified locally: after dropping System.Runtime.CompilerServices.Unsafe.dll v6
into the plugins folder and forcing a domain reload, the execute_code tool
compiles C# 7+ snippets via the Roslyn backend successfully.

Author: sMartz1

MCPForUnity/Editor/Setup/RoslynInstaller.cs

@@ -13,10 +13,14 @@ public static class RoslynInstaller
 
         private static readonly (string packageId, string version, string dllPath, string dllName)[] NuGetEntries =
         {
-            ("microsoft.codeanalysis.common",    "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.dll",       "Microsoft.CodeAnalysis.dll"),
-            ("microsoft.codeanalysis.csharp",    "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.CSharp.dll","Microsoft.CodeAnalysis.CSharp.dll"),
-            ("system.collections.immutable",     "8.0.0",  "lib/netstandard2.0/System.Collections.Immutable.dll", "System.Collections.Immutable.dll"),
-            ("system.reflection.metadata",       "8.0.0",  "lib/netstandard2.0/System.Reflection.Metadata.dll",   "System.Reflection.Metadata.dll"),
+            ("microsoft.codeanalysis.common",         "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.dll",                   "Microsoft.CodeAnalysis.dll"),
+            ("microsoft.codeanalysis.csharp",         "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.CSharp.dll",            "Microsoft.CodeAnalysis.CSharp.dll"),
+            ("system.collections.immutable",          "8.0.0",  "lib/netstandard2.0/System.Collections.Immutable.dll",             "System.Collections.Immutable.dll"),
+            ("system.reflection.metadata",            "8.0.0",  "lib/netstandard2.0/System.Reflection.Metadata.dll",               "System.Reflection.Metadata.dll"),
+            // Transitive dep of Microsoft.CodeAnalysis.* on netstandard2.0. Without it, Roslyn's StringTable
+            // static cctor throws FileNotFoundException for v6.0.0.0 and every Roslyn entry point fails to
+            // initialize. Unity ships a v4.x of this assembly which does NOT satisfy the v6 reference.
+            ("system.runtime.compilerservices.unsafe","6.0.0",  "lib/netstandard2.0/System.Runtime.CompilerServices.Unsafe.dll",   "System.Runtime.CompilerServices.Unsafe.dll"),
         };
 
         public static bool IsInstalled()

MCPForUnity/Editor/Tools/ExecuteCode.cs

@@ -672,7 +672,13 @@ public static Assembly Compile(string source, string[] assemblyPaths, out List<s
             }
             catch (Exception e)
             {
-                errors.Add($"Roslyn compilation error: {e.Message}");
+                // Walk to the deepest cause: TargetInvocationException (and friends) wrap the real
+                // failure inside .InnerException, and reporting only e.Message hides everything
+                // useful (e.g. a missing transitive dep manifests as the generic "Exception has been
+                // thrown by the target of an invocation.").
+                Exception root = e;
+                while (root.InnerException != null) root = root.InnerException;
+                errors.Add($"Roslyn compilation error: {root.GetType().Name}: {root.Message}");
                 return null;
             }
         }